For LCD devices, a classification based on the operating mode of liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a field-induced photo-reactive alignment (FPA) mode. A classification based on the driving mode of the device includes passive matrix (PM) type and active matrix (AM) type. The PM type is further classified into static type, multiplex type and so forth, and the AM type is classified into thin film transistor (TFT) type, metal insulator metal (MIM) type and so on. The TFT type is further classified into amorphous silicon type and polysilicon type. The latter is classified into a high temperature type and a low temperature type according to the production process. A classification based on the light source includes a reflective type utilizing natural light, a transmissive type utilizing a backlight and a transflective type utilizing both the natural light and the backlight.
The LCD device includes a liquid crystal composition having a nematic phase. The composition has suitable characteristics. An AM device having good characteristics can be obtained by improving the characteristics of the composition. Table 1 below summarizes a relationship between the characteristics of the two aspects. The characteristics of the composition will be further described based on a commercially available AM device. The temperature range of the nematic phase relates to the temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is about 70° C. or higher and a preferred minimum temperature of the nematic phase is about −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is preferred for displaying moving images on the device. A shorter response time even by one millisecond is desirable. Accordingly, a small viscosity of the composition is preferred. A small viscosity at a low temperature is further preferred.
TABLE 1General Characteristics of Composition and AM DeviceGeneral CharacteristicsGeneral Characteristics No.of Compositionof AM Device1 Wide temperature range Wide usable of a nematic phasetemperature range2Small viscosityShort response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negative Low threshold voltage anddielectric anisotropysmall electric power consumption,Large contrast ratio5Large specific resistanceLarge voltage holding ratio and large contrast ratio6High stability to Long service lifeUV light and heat
The optical anisotropy of the composition relates to the contrast ratio of the device. According to the mode of the device, a large optical anisotropy or a small optical anisotropy, namely a suitable optical anisotropy, is required. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the type of the operating mode. The value is in the range of about 0.30 μm to about 0.40 μm in a device of the VA mode, and in the range of about 0.20 μm to about 0.30 μm in a device of the IPS or FFS mode. In the above cases, a composition having a large Δn is preferred for a device having a small cell gap. A large dielectric anisotropy (Δ∈) of the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio of the device. Accordingly, a large Δ∈ is preferred. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance at room temperature and also at a high temperature in an initial stage is preferred. A composition having a large specific resistance at room temperature and also at a high temperature even after the device has been used for a long period of time is preferred. Stability of the composition to UV light and heat relates to a service life of the device. In the case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device for use in a liquid crystal projector, a liquid crystal television and so forth.
A liquid crystal composition containing a polymer is used for a PSA-mode LCD device. First, a composition to which a small amount of a polymerizable compound is added is injected into a device. Next, the composition is irradiated with UV light, while voltage is applied between substrates of the device, to polymerize the polymerizable compound and produce a polymer network structure in the composition. In the composition, alignment of liquid crystal molecules can be controlled by the polymer. Thus, the response time of the device is shortened and image persistence is improved. Such an effect of the polymer can be expected for the device that has the mode such as TN, ECB, OCB, IPS, VA, FFS or FPA.
A composition having a positive Δ∈ is used for an AM device of the TN mode. A composition having a negative Δ∈ is used for an AM device of the VA mode. A composition having a positive or negative Δ∈ is used for an AM device of the IPS or FFS mode. A composition having a positive or negative Δ∈ is used for a PSA-mode AM device. Examples of the liquid crystal composition having a negative Δ∈ are disclosed in Patent literature Nos. 1 to 10 described below.    Patent literature No. 1: JP 2000-53602 A.    Patent literature No. 2: JP 2001-115161 A.    Patent literature No. 3: WO 2008/114821 A.    Patent literature No. 4: WO 2009/157313 A.    Patent literature No. 5: WO 2010/16387 A.    Patent literature No. 6: WO 2010/84810 A.    Patent literature No. 7: WO 2011/40170 A.    Patent literature No. 8: WO 2011/152494 A.    Patent literature No. 9: WO 2012/53323 A.    Patent literature No. 10: WO 2008/114779 A.